1. Field of the Invention
The present invention relates to a semiconductor sensor devices and, more particularly, to a method of forming a semiconductor sensor device using MEMS technology.
2. Description of the Related Art
A micro-electromechanical system (MEMS) is a microscopic machine that is fabricated using the same types of steps (e.g., the deposition of layers of material and the selective removal of the layers of material) that are used to fabricate conventional analog and digital CMOS circuits.
One type of MEMS is a pressure sensor. Pressure sensors commonly use a diaphragm, and measure pressure by measuring changes in the deformation of the diaphragm. The deformation of the diaphragm, in turn, can be detected by changes in the capacitance or piezoresistance of the diaphragm.
FIGS. 1A–1B show views that illustrate a prior-art, piezoresistive MEMS pressure sensor 100. FIG. 1A shows a plan view, while FIG. 1B shows a cross-sectional view taken along line 1B—1B of FIG. 1A. As shown in FIGS. 1A–1B, MEMS pressure sensor 100 includes a p-base 110 and a p-diaphragm 112 that is supported by base 110.
In addition, MEMS pressure sensor 100 also includes four piezoresistors R1, R2, R3, and R4 that are formed in diaphragm 112 and electrically connected together. For example, the piezoresistors R1, R2, R3, and R4 can be implemented with n+ doped regions, which are each aligned with the <110> crystal orientation.
As further shown in FIGS. 1A–1B, the piezoresistors R1, R2, R3, and R4 are connected in a Wheatstone Bridge configuration where a sense voltage VS is placed between the node located between resistors R2 and R3, and the node located between resistors R1 and R4. Further, an output voltage VO is taken between the node located between resistors R1 and R2, and the node located between resistors R3 and R4.
In operation, when the pressure changes, the change in pressure changes the strain placed on the piezoresistors R1, R2, R3, and R4. The change in strain deforms the band gap structures of the piezoresistors R1, R2, R3, and R4. The deformed band gap structures change the mobility and density of the charge carriers which, in turn, changes the resistivity. The changes in resistivity are detected by the Wheatstone Bridge, which varies the output voltage VO in response to the changes in resistivity.
MEMS pressure sensor 100 is typically quite complex to fabricate. For example, MEMS pressure sensor 100 can be fabricated by forming the diaphragm over a sacrificial material, and then etching away the sacrificial material to expose the diaphragm. Alternately, the diaphragm can be exposed by etching in from the backside.
Thus, although there are a number of ways of fabricating MEMS pressure sensors, there is a need for a less expensive approach to the fabrication of a MEMS pressure sensor.